The Cataclysm

Over two thousand years ago, the world stood on a razors edge and teetered over the abyss. Destruction ran rampant as demon hordes poured from the planar gate, opened through great magics turned treacherous. Their unstoppable hordes sought to remake hell upon the mortal realm, devouring life and souls as they went.
Only the combined might of all the mortal races managed to push back the tide. Years of fighting and millions of deaths brought the united armies, at last to the great gate. The once majestic city around it despoiled and corrupted by the very creatures themselves, so much so that many stalwart soldiers were driven mad on the final approach.
At last the gate was reached and the most foul servants of hell and beyond awaited the mortal armies. The battle would be the last of the great war and fates worse than death greeted those that fell that day. At the end a score of great wizards and sorcerers reached the foot of the gate and through the use of powerful artefacts managed to close the gate, halting the flood of demons for the first time in years. The silence that followed was deafening as if the world stopped spinning for just a moment and a demon god drew in his breath. The next sound split the world as the demon god's scream of rage at his lost prize shattered the earth and all who stood upon it.
The gate and its magics detonated like an eruption, dragging the land beneath the ocean. All within a hundred miles were vaporised in the initial blast that triggered great tsunamis and volcanic eruptions around the world. A full half of the continent sank below the sea as did the souls of those who had given everything to save the world.
The now split continents would never again be joined and those upon them would forget their own history, too eager to seize power from the ashes. The mortal races soon began to bicker and fight amongst themselves, seeking ancient artefacts of power and materials, forgetting those who gave everything for them. History would fall into legend, too soon forgotten, for the horrors of the beyond would never forget. Their grip ever tightening on the world once again.